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Landscape ecology and stability of populations
Landscape Planning, 4 (1977) 85-93
85
'~© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
LANDSCAPE ECOLOGY AND STABILITY OF POPULATIONS
LENNART HANSSON
Department of Vertebrate Ecology, Swedish Agricultural University, 5-750 07 Uppsala (Sweden) (Received 21 July 1976)
ABSTRACT Hansson, L ., 1977 . Landscape ecology and stability of populations . Landscape Plann ., 4 : 85-93 . An attempt is made to widen the concept of "landscape ecology" to include effects of the structural composition of landscapes on the abundance of animal and plant populations inside separate ecosystems . This composition affects species living in different habitats during the day or the year, species which are exposed to predation or competition from species living in other ecosystems, and generalist species, able to live and reproduce in various ecosystems . Possible research approaches are examined, including experiments, regression models, expanded ecosystem models and productivity studies . As an example, the use of the landscape by the field vole, a forest pest species, is outlined . It is concluded that some recent large-scale damage by this species may be due to an unfortunate structuring of the landscape . Finally it is pointed out that the importance of the landscape structure should be considered both for conservation of endangered species and for prevention of pest outbreak.
INTRODUCTION
Ecologists at present focus their interest on ecosystems (biotic communities of plants, animals and microorganisms and their abiotic environment) . However, it is fully realized that almost all ecosystems are "open", i .e. there is a movement of organisms, energy and matter out of or into the system (Odum, 1971, gives some excellent examples of this) . Another unit in the hierarchy of organic communities is the biome (physiognomically and environmentally similar complexes of biotic communities, e .g . the deciduous forest, in different parts of the world), for which there is an increasing interest (see Whittaker, 1972) . However, in between ecosystems and biome is the landscape, which deserves closer attention than it receives today . We shall discuss here the relations between the composition of the landscape and the number of animals or plants of various species in particular areas . For an ecologist the term landscape means the mosaic of, relative size of, and distances between, various ecosystems or habitats in a limited region .
86 Habitats are the biotic and abiotic environments of certain species . Thus, landscape effects may be studied from the point of view of both the separate ecosystem unit and the separate species population . It is suggested that such interrelations are important and that changes in abundance in stable ecosystems may be caused by structural alterations in the landscape . Such effects may vary from the extinction of species to pest outbreaks, i .e . the stability of the populations is affected . In this context "stability" refers to long-term constancy levels in the numbers of plants and animals . Thus, the landscape composition may act as a limiting factor on population density in some of the several ecosystems . In this paper there is an attempt to widen the concept of "landscape ecology" . This concept has hitherto mainly been used in landscape architecture and rural planning . However, it may also he of fundamental importance in population ecology . TYPES OF LANDSCAPE INFLUENCES There are at least four different ways in which the landscape mosaic influences population numbers . These influences may also interact in various ways . Use of landscape complexes in daily life Many animal species, perhaps mainly the larger animals, e .g. vertebrates, have evolved the ability to live in two or more ecosystems . Many others have perhaps evolved within a restricted ecosystem but have also learned to make use of new ecosystems, especially those arising from human cultivation . At present it may be difficult to separate an evolved innate adaptation and a learned acclimatization to landscape mosaics (i .e . an array of structurally different habitats) . Examples of such animals are crows, roosting in winter in forests but searching for food on farmland or even in towns, several birds of prey (buzzards, kites) nesting in forests but hunting in open areas including farmland, sea shores and also the open sea (sea eagles) . Some deer species stay mainly in forests but use open fields for feeding at night . Similar behaviour is found in foxes and other mammalian predators . Seasonal changes of habitat selection The changes we are concerned with here occur when one part of the life cycle is completed in one ecosystem, and other parts or the rest in other ecosystems . These adaptations occur mainly in animals but some plant species, e .g . host-alternating parasitic fungi, may also be considered in this context . Examples among the lower animals are host-changing aphids and many in-
87
sects (mosquitoes, dragon-flies etc .) with larval stages in lakes and rivers and an adult stage in a terrestrial ecosystem . Frogs and toads spawn in ponds but live as adults during the main part of their life cycle in completely different ecosystems . In summer bats may live in open areas or forests and hibernate in winter in caves or buildings at various distances from the summer habitat, but their appearance is certainly influenced by the distance between the two places . Extreme cases are migratory fish and birds and some mammals e .g . reindeer, where the movements are usually not between ecosystems in the same landscape, but rather between biomes .
Interactions from other ecosystems It was mentioned that grazing animals, such as deer, and predators such as buzzards and foxes often find their food in ecosystems other than those in which they breed and find shelter . Thus, proximity to forest areas may affect plant food species, e .g . various herbs, or prey such as rodents thriving in open field habitats . This means that species which complete their entire life cycle within one ecosystem may still be profoundly affected by the structural composition of the landscape . A similar situation is the movement of a competing species into several ecosystems . However, our knowledge of competition is still scanty and there are no clear examples .
Expansion of generalist species Some species show adaptation to a broad set of ecosystems, but live and breed more successfully in one optimum type . A common type of generalist species is characterized by a high reproductive performance but a generally low survival rate . Such species are called opportunists and are potential pest species . If in a particular landscape there exists an optimum ecosystem and some other ecosystems which can marginally support such opportunist populations, the distance between areas may be decisive for population sizes . From the optimum area (a donor habitat) animals may, on dispersal, continuously enter other possible breeding areas (reception habitats) and there suffer a higher mortality rate . The influence of the donor habitat will be weaker the further the dispersing animals have to travel through non-supporting (transition) habitats. Thus when this distance is shortened, e .g . in the clearing of forests, opportunist species may increase in number and eventually reach pest proportions . The suitability of different habitats for animal generalists may depend on innate, behavioural reactions to ecosystem structures . However, there might also be equally important differences in exposure to weather, nutrition, predation and competition (Hansson, 1975) . The latter conditions also apply to
SB generalist plant populations, of which opportunistic weeds are typical . One well-known example is the hark-beetle, for which storm-felling of forests may create temporarily optimal habitats, from where the dispersing animals then invade reception habitats in the form of standing living trees, with the result that a disastrous pest outbreak and economic destruction occurs . Sometimes breeding or non-breeding generalist populations overexploit a habitat . Weather conditions may also make feeding difficult or impossible . Parts of or even whole populations may move to other ecosystems (which may be transitional) and sometimes cause considerable damage . An example is that of hares and rabbits feeding on fruit-trees in gardens, when the natural food vegetation is snow-coveredRESEARCH APPROACHES The conditions described above have long been well known to naturalists . However, there are few if any quantitative studies on the influence of the landscape composition . Some suggestions for this type of research will be given here . However, more rigorous mathematical treatments will have to he developed in other areas . Experiments Unambigous evidence of the effects of various landscape structures will be provided by experiments . It is, however, very difficult to carry out complete experiments on landscapes . It might he possible to follow changes in population numbers in untouched ecosystems when other neighbouring ecosystems are changed by human interference, but the latter changes are seldom distributed randomly . Thus, such studies have to be evaluated carefully . However, habitats may sometimes be rendered uninhabitable for some animals or plant species without destruction of the whole ecosystem . For example, burning and the use of herbicides in a forest might only destroy the field layer without affecting the trees . Such treatment within conventional experimental designs might be one of the best ways of studying the effects of landscape on populations . Surveys and multiple regression models Populations numbers can be sampled in the various habitats frequented by a species . The numbers obtained at sample points, and corresponding landscape composition variables may be included in a larger regression model that also includes other variables known to affect the species . Structural elements of the landscape might be defined as the percentage of open fields, distances to optimal habitats or breeding places, etc . Other important parameters may be percentage cover, tree height, food abundance etc . By removing variation
89
due to these latter parameters it might be possible to establish underlying correlation with landscape structure . Total system studies When the landscape effect is a result of complex interactions between factors such as food, competition and predators, studies of all these components will be profitable but tedious . It means that predators for example will have to be studied in some ecosystem other than that where the species under investigation is living . This involves an extension of present large ecosystem studies, but only a few clearly interacting variables may need to be studied in detail . Studies on productivity and dispersal Productivity investigations can be a partial substitute for system studies, although all causal relations will not be apparent . Food supply, competition, predation, etc . during reproductive seasons will influence the production of young (or diaspores) . Most animals have a dispersal stage either just before, or at, maturation . It might, therefore, be possible to estimate the importance of various landscapes for a certain species on the basis of the production of animals at the dispersal stage, taking into account immigration, and young recruited in the habitat where they were born . In this way various habitats may be quantitatively characterized by, for example, dispersing animals/ha/day or dispersing animals/ha/ breeding season . Ecosystems with a positive population balance even when production by settled immigrants is ignored, are primary donor habitats, and as such affect the total population in the landscape . Another important factor is the survival rate per unit distance in transition habitats . This factor will determine how many immigrants will reach reception habitats (which have a negative balance per se) and thus decide whether these areas will contribute further to the expansion of the population . Studies on migrations of the adults of the populations outside the breeding period may provide evidence of unfavourable conditions in the habitats . Such studies may also be quantitative, and demonstrate the importance of invasions between ecosystems in the same landscape . Studies on animals may be carried out by live-trapping and tagging, isotopes, telemetry, etc . For plants, isotopes may be the best means .
EXAMPLE : THE POPULATION EXPANSION OF FIELD VOLES (MICROTUS AGRESTIS L .) The fields vole is an interesting species in connection with landscape ecology . It changes its habitat on a partly seasonal basis, breeds in several habitats, is a
90
typical opportunist and a pest species particularly in forestry . Its habitats arc well-known from studies in southern Sweden (Hansson, 1971) and northern Sweden (Hansson, unpublished data) and in southern Finland (Myllymaki, 1970, 1975) . However, these studies have mainly been performed from an ecosystem point of view and the landscape effects can only be inferred . The latter may, however, turn out to be of considerable practical importance .
10021*
Donor
Reception habitat
Induced donor habitat
Transition habitat
)
Fig .1 . An example of the effects of landscape structure on the spatial dynamics of an unspecialized species, the field vole Microtus agre.siis . A . A landscape with low production of voles . B . A landscape with high production of voles .
91
The preferred habitats of the field vole are open fields with a luxuriant vegetation, where the soil is covered with a layer of dead vegetation (litter) . Such areas are most common where the ground water level is close to the top-soil . Most of these habitats are therefore flooded seasonally, in southern Sweden often between October and March, in northern Fennoscandia during autumn and spring . When there is a cover of ice and snow, flooded areas may be repopulated in winter . Population movements have also been found in drier areas in winter but they are still poorly understood . However, some abandoned fields, dominated by tussocks of the grass Deschampsia cespitosa and covered with ground litter are not usually flooded, and form a permanent optimum habitat . Lower population densities are usually found on dry disused fields (e.g. on sandy soil with little shelter), leys (only temporary habitation with breeding populations in summer) and of reafforestation areas with a dense mat of the grass Deschampsia flexuosa . Transition habitats are occupied in summer, for example, forests and cultivated fields with the exception of leys . From the landscape ecology point of view the two following questions, among others, can he raised : (1) Important damage to conifer seedlings appears in large reafforestation areas in central and northern Sweden . Do neighbouring disused fields, which have changed through secondary successions to D. cespitosa areas, constitute donor habitats, which support field vole populations on nearby reafforestation areas? (2) In large (50-200 ha) reafforestation areas, especially those close to small lakes and mires, luxuriant "meadows" are developed with groundcovering vegetation . Do these areas form explosive donor habitats, from which breeding animals are easily recruited to the nearby drier parts of the reafforestation area? That is, does the size of the reafforestation areas indirectly affect the density of field vole populations and the damage done by them? The first problem may be solved by experiment . It may be possible to destroy such habitats randomly by spring application of soil herbicides and summer burning, and then to estimate the numbers and reproduction rate in nearby reafforestation areas . The second problem is probably more easily approached by productivity studies . The animals are trapped alive and tagged in the "wet meadows" and in the D. flexuosa mats before dispersal starts . The fate of dispersing voles is traced by trapping lines and population balances computed for the two types of habitat . An alternative method is to sample field voles randomly in the D. flexuosa mats in reafforestation areas of various sizes, and then to enter the numbers obtained into a multiple regression model . One variable in this model would be the size of the reafforestation area . A significant correlation with size would merit further study . Both types of investigation would have to be repeated several times during the cycles of abundance, which are characteristic of many vole populations .
92 APPLICATION OF RESEARCH RESULTS
Knowledge of landscape effects may be utilized practically at least in two important respects : (a) For the conservation of endangered species it is usually not enough to protect one ecosystem, but rather an array of habitats should be considered .
Fig.2 . A recently cleared forest area in northern Sweden . The reafforestation area includes some wet parts, which will soon develop a luxuriant field layer vegetation . Older clearings were smaller and did not include mires or lake shores .
Fig .3 . From a luxuriant grass vegetation on a smaller part of a reafforested area ("a grassland"), field voles may enter remaining areas (e .g . "woodlands") and cause serious damage by debarking the tree seedlings .
93
Better knowledge will make it possible to delimit more precisely the important areas and to guide management. (b) Pest outbreaks may be directly related to the habitat composition of the landscape . With better insight, potential outbreak areas may be recognized and control measures confined to specific geographical regions . Sometimes a special ecosystem within a landscape deserves attention . It is often difficult or impossible to change landscape structures just for the purpose of influencing pest populations . However, as an example the landscape structures of the Fennoscandian coniferous forests are now undergoing a complete transformation and there may be a good opportunity in this example to apply future ecological principles on the influences of landscape on pest populations .
REFERENCES Hansson, L ., 1971 . Habitat, food and population dynamics of the field vole Microtus agrestis (L.) in south Sweden . Viltrevy, 8 : 267-378 . Hansson, L ., 1975 . Effects of habitat manipulation on small rodent populations . Ecol . Bull., 19 : 163-173 . Myllymaki, A., 1970 . Population ecology and its application to the control of the field vole, Microtus agrestis (L.). EPPO Publ . Ser . A, 58 : 27-48 . Myllymaki, A., 1975 . Outbreaks and damage by field rodents and other harmful small mammals in Finland . Ecol . Bull ., 19 : 17-36 . Odum, E .P ., 1971 . Fundamentals of Ecology . W .B . Saunders, Philadelphia, Pa ., 574 pp . Whittaker, R.H ., 1972 . Communities and Ecosystems . MacMillan, London, 158 pp .
85
'~© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
LANDSCAPE ECOLOGY AND STABILITY OF POPULATIONS
LENNART HANSSON
Department of Vertebrate Ecology, Swedish Agricultural University, 5-750 07 Uppsala (Sweden) (Received 21 July 1976)
ABSTRACT Hansson, L ., 1977 . Landscape ecology and stability of populations . Landscape Plann ., 4 : 85-93 . An attempt is made to widen the concept of "landscape ecology" to include effects of the structural composition of landscapes on the abundance of animal and plant populations inside separate ecosystems . This composition affects species living in different habitats during the day or the year, species which are exposed to predation or competition from species living in other ecosystems, and generalist species, able to live and reproduce in various ecosystems . Possible research approaches are examined, including experiments, regression models, expanded ecosystem models and productivity studies . As an example, the use of the landscape by the field vole, a forest pest species, is outlined . It is concluded that some recent large-scale damage by this species may be due to an unfortunate structuring of the landscape . Finally it is pointed out that the importance of the landscape structure should be considered both for conservation of endangered species and for prevention of pest outbreak.
INTRODUCTION
Ecologists at present focus their interest on ecosystems (biotic communities of plants, animals and microorganisms and their abiotic environment) . However, it is fully realized that almost all ecosystems are "open", i .e. there is a movement of organisms, energy and matter out of or into the system (Odum, 1971, gives some excellent examples of this) . Another unit in the hierarchy of organic communities is the biome (physiognomically and environmentally similar complexes of biotic communities, e .g . the deciduous forest, in different parts of the world), for which there is an increasing interest (see Whittaker, 1972) . However, in between ecosystems and biome is the landscape, which deserves closer attention than it receives today . We shall discuss here the relations between the composition of the landscape and the number of animals or plants of various species in particular areas . For an ecologist the term landscape means the mosaic of, relative size of, and distances between, various ecosystems or habitats in a limited region .
86 Habitats are the biotic and abiotic environments of certain species . Thus, landscape effects may be studied from the point of view of both the separate ecosystem unit and the separate species population . It is suggested that such interrelations are important and that changes in abundance in stable ecosystems may be caused by structural alterations in the landscape . Such effects may vary from the extinction of species to pest outbreaks, i .e . the stability of the populations is affected . In this context "stability" refers to long-term constancy levels in the numbers of plants and animals . Thus, the landscape composition may act as a limiting factor on population density in some of the several ecosystems . In this paper there is an attempt to widen the concept of "landscape ecology" . This concept has hitherto mainly been used in landscape architecture and rural planning . However, it may also he of fundamental importance in population ecology . TYPES OF LANDSCAPE INFLUENCES There are at least four different ways in which the landscape mosaic influences population numbers . These influences may also interact in various ways . Use of landscape complexes in daily life Many animal species, perhaps mainly the larger animals, e .g. vertebrates, have evolved the ability to live in two or more ecosystems . Many others have perhaps evolved within a restricted ecosystem but have also learned to make use of new ecosystems, especially those arising from human cultivation . At present it may be difficult to separate an evolved innate adaptation and a learned acclimatization to landscape mosaics (i .e . an array of structurally different habitats) . Examples of such animals are crows, roosting in winter in forests but searching for food on farmland or even in towns, several birds of prey (buzzards, kites) nesting in forests but hunting in open areas including farmland, sea shores and also the open sea (sea eagles) . Some deer species stay mainly in forests but use open fields for feeding at night . Similar behaviour is found in foxes and other mammalian predators . Seasonal changes of habitat selection The changes we are concerned with here occur when one part of the life cycle is completed in one ecosystem, and other parts or the rest in other ecosystems . These adaptations occur mainly in animals but some plant species, e .g . host-alternating parasitic fungi, may also be considered in this context . Examples among the lower animals are host-changing aphids and many in-
87
sects (mosquitoes, dragon-flies etc .) with larval stages in lakes and rivers and an adult stage in a terrestrial ecosystem . Frogs and toads spawn in ponds but live as adults during the main part of their life cycle in completely different ecosystems . In summer bats may live in open areas or forests and hibernate in winter in caves or buildings at various distances from the summer habitat, but their appearance is certainly influenced by the distance between the two places . Extreme cases are migratory fish and birds and some mammals e .g . reindeer, where the movements are usually not between ecosystems in the same landscape, but rather between biomes .
Interactions from other ecosystems It was mentioned that grazing animals, such as deer, and predators such as buzzards and foxes often find their food in ecosystems other than those in which they breed and find shelter . Thus, proximity to forest areas may affect plant food species, e .g . various herbs, or prey such as rodents thriving in open field habitats . This means that species which complete their entire life cycle within one ecosystem may still be profoundly affected by the structural composition of the landscape . A similar situation is the movement of a competing species into several ecosystems . However, our knowledge of competition is still scanty and there are no clear examples .
Expansion of generalist species Some species show adaptation to a broad set of ecosystems, but live and breed more successfully in one optimum type . A common type of generalist species is characterized by a high reproductive performance but a generally low survival rate . Such species are called opportunists and are potential pest species . If in a particular landscape there exists an optimum ecosystem and some other ecosystems which can marginally support such opportunist populations, the distance between areas may be decisive for population sizes . From the optimum area (a donor habitat) animals may, on dispersal, continuously enter other possible breeding areas (reception habitats) and there suffer a higher mortality rate . The influence of the donor habitat will be weaker the further the dispersing animals have to travel through non-supporting (transition) habitats. Thus when this distance is shortened, e .g . in the clearing of forests, opportunist species may increase in number and eventually reach pest proportions . The suitability of different habitats for animal generalists may depend on innate, behavioural reactions to ecosystem structures . However, there might also be equally important differences in exposure to weather, nutrition, predation and competition (Hansson, 1975) . The latter conditions also apply to
SB generalist plant populations, of which opportunistic weeds are typical . One well-known example is the hark-beetle, for which storm-felling of forests may create temporarily optimal habitats, from where the dispersing animals then invade reception habitats in the form of standing living trees, with the result that a disastrous pest outbreak and economic destruction occurs . Sometimes breeding or non-breeding generalist populations overexploit a habitat . Weather conditions may also make feeding difficult or impossible . Parts of or even whole populations may move to other ecosystems (which may be transitional) and sometimes cause considerable damage . An example is that of hares and rabbits feeding on fruit-trees in gardens, when the natural food vegetation is snow-coveredRESEARCH APPROACHES The conditions described above have long been well known to naturalists . However, there are few if any quantitative studies on the influence of the landscape composition . Some suggestions for this type of research will be given here . However, more rigorous mathematical treatments will have to he developed in other areas . Experiments Unambigous evidence of the effects of various landscape structures will be provided by experiments . It is, however, very difficult to carry out complete experiments on landscapes . It might he possible to follow changes in population numbers in untouched ecosystems when other neighbouring ecosystems are changed by human interference, but the latter changes are seldom distributed randomly . Thus, such studies have to be evaluated carefully . However, habitats may sometimes be rendered uninhabitable for some animals or plant species without destruction of the whole ecosystem . For example, burning and the use of herbicides in a forest might only destroy the field layer without affecting the trees . Such treatment within conventional experimental designs might be one of the best ways of studying the effects of landscape on populations . Surveys and multiple regression models Populations numbers can be sampled in the various habitats frequented by a species . The numbers obtained at sample points, and corresponding landscape composition variables may be included in a larger regression model that also includes other variables known to affect the species . Structural elements of the landscape might be defined as the percentage of open fields, distances to optimal habitats or breeding places, etc . Other important parameters may be percentage cover, tree height, food abundance etc . By removing variation
89
due to these latter parameters it might be possible to establish underlying correlation with landscape structure . Total system studies When the landscape effect is a result of complex interactions between factors such as food, competition and predators, studies of all these components will be profitable but tedious . It means that predators for example will have to be studied in some ecosystem other than that where the species under investigation is living . This involves an extension of present large ecosystem studies, but only a few clearly interacting variables may need to be studied in detail . Studies on productivity and dispersal Productivity investigations can be a partial substitute for system studies, although all causal relations will not be apparent . Food supply, competition, predation, etc . during reproductive seasons will influence the production of young (or diaspores) . Most animals have a dispersal stage either just before, or at, maturation . It might, therefore, be possible to estimate the importance of various landscapes for a certain species on the basis of the production of animals at the dispersal stage, taking into account immigration, and young recruited in the habitat where they were born . In this way various habitats may be quantitatively characterized by, for example, dispersing animals/ha/day or dispersing animals/ha/ breeding season . Ecosystems with a positive population balance even when production by settled immigrants is ignored, are primary donor habitats, and as such affect the total population in the landscape . Another important factor is the survival rate per unit distance in transition habitats . This factor will determine how many immigrants will reach reception habitats (which have a negative balance per se) and thus decide whether these areas will contribute further to the expansion of the population . Studies on migrations of the adults of the populations outside the breeding period may provide evidence of unfavourable conditions in the habitats . Such studies may also be quantitative, and demonstrate the importance of invasions between ecosystems in the same landscape . Studies on animals may be carried out by live-trapping and tagging, isotopes, telemetry, etc . For plants, isotopes may be the best means .
EXAMPLE : THE POPULATION EXPANSION OF FIELD VOLES (MICROTUS AGRESTIS L .) The fields vole is an interesting species in connection with landscape ecology . It changes its habitat on a partly seasonal basis, breeds in several habitats, is a
90
typical opportunist and a pest species particularly in forestry . Its habitats arc well-known from studies in southern Sweden (Hansson, 1971) and northern Sweden (Hansson, unpublished data) and in southern Finland (Myllymaki, 1970, 1975) . However, these studies have mainly been performed from an ecosystem point of view and the landscape effects can only be inferred . The latter may, however, turn out to be of considerable practical importance .
10021*
Donor
Reception habitat
Induced donor habitat
Transition habitat
)
Fig .1 . An example of the effects of landscape structure on the spatial dynamics of an unspecialized species, the field vole Microtus agre.siis . A . A landscape with low production of voles . B . A landscape with high production of voles .
91
The preferred habitats of the field vole are open fields with a luxuriant vegetation, where the soil is covered with a layer of dead vegetation (litter) . Such areas are most common where the ground water level is close to the top-soil . Most of these habitats are therefore flooded seasonally, in southern Sweden often between October and March, in northern Fennoscandia during autumn and spring . When there is a cover of ice and snow, flooded areas may be repopulated in winter . Population movements have also been found in drier areas in winter but they are still poorly understood . However, some abandoned fields, dominated by tussocks of the grass Deschampsia cespitosa and covered with ground litter are not usually flooded, and form a permanent optimum habitat . Lower population densities are usually found on dry disused fields (e.g. on sandy soil with little shelter), leys (only temporary habitation with breeding populations in summer) and of reafforestation areas with a dense mat of the grass Deschampsia flexuosa . Transition habitats are occupied in summer, for example, forests and cultivated fields with the exception of leys . From the landscape ecology point of view the two following questions, among others, can he raised : (1) Important damage to conifer seedlings appears in large reafforestation areas in central and northern Sweden . Do neighbouring disused fields, which have changed through secondary successions to D. cespitosa areas, constitute donor habitats, which support field vole populations on nearby reafforestation areas? (2) In large (50-200 ha) reafforestation areas, especially those close to small lakes and mires, luxuriant "meadows" are developed with groundcovering vegetation . Do these areas form explosive donor habitats, from which breeding animals are easily recruited to the nearby drier parts of the reafforestation area? That is, does the size of the reafforestation areas indirectly affect the density of field vole populations and the damage done by them? The first problem may be solved by experiment . It may be possible to destroy such habitats randomly by spring application of soil herbicides and summer burning, and then to estimate the numbers and reproduction rate in nearby reafforestation areas . The second problem is probably more easily approached by productivity studies . The animals are trapped alive and tagged in the "wet meadows" and in the D. flexuosa mats before dispersal starts . The fate of dispersing voles is traced by trapping lines and population balances computed for the two types of habitat . An alternative method is to sample field voles randomly in the D. flexuosa mats in reafforestation areas of various sizes, and then to enter the numbers obtained into a multiple regression model . One variable in this model would be the size of the reafforestation area . A significant correlation with size would merit further study . Both types of investigation would have to be repeated several times during the cycles of abundance, which are characteristic of many vole populations .
92 APPLICATION OF RESEARCH RESULTS
Knowledge of landscape effects may be utilized practically at least in two important respects : (a) For the conservation of endangered species it is usually not enough to protect one ecosystem, but rather an array of habitats should be considered .
Fig.2 . A recently cleared forest area in northern Sweden . The reafforestation area includes some wet parts, which will soon develop a luxuriant field layer vegetation . Older clearings were smaller and did not include mires or lake shores .
Fig .3 . From a luxuriant grass vegetation on a smaller part of a reafforested area ("a grassland"), field voles may enter remaining areas (e .g . "woodlands") and cause serious damage by debarking the tree seedlings .
93
Better knowledge will make it possible to delimit more precisely the important areas and to guide management. (b) Pest outbreaks may be directly related to the habitat composition of the landscape . With better insight, potential outbreak areas may be recognized and control measures confined to specific geographical regions . Sometimes a special ecosystem within a landscape deserves attention . It is often difficult or impossible to change landscape structures just for the purpose of influencing pest populations . However, as an example the landscape structures of the Fennoscandian coniferous forests are now undergoing a complete transformation and there may be a good opportunity in this example to apply future ecological principles on the influences of landscape on pest populations .
REFERENCES Hansson, L ., 1971 . Habitat, food and population dynamics of the field vole Microtus agrestis (L.) in south Sweden . Viltrevy, 8 : 267-378 . Hansson, L ., 1975 . Effects of habitat manipulation on small rodent populations . Ecol . Bull., 19 : 163-173 . Myllymaki, A., 1970 . Population ecology and its application to the control of the field vole, Microtus agrestis (L.). EPPO Publ . Ser . A, 58 : 27-48 . Myllymaki, A., 1975 . Outbreaks and damage by field rodents and other harmful small mammals in Finland . Ecol . Bull ., 19 : 17-36 . Odum, E .P ., 1971 . Fundamentals of Ecology . W .B . Saunders, Philadelphia, Pa ., 574 pp . Whittaker, R.H ., 1972 . Communities and Ecosystems . MacMillan, London, 158 pp .